Numerous detector devices, and particularly alarms, make use of ultrasonics, i.e. very high frequencies. These devices have both advantages and disadvantages. The disadvantages include the possibility of accidental triggering and poor detector specificity, gradual detection being difficult. Consequently, ultrasonic or very high frequency detectors are used to provide protection for an empty room or flat in which nothing can move whilst this type of detector is in operation without the movement being detected.
This invention aims to alleviate or overcome the disadvantages noted above.